In Situ Raman Microscopy of Chromate Effects on Corrosion Pits in Aluminum Alloy

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Abstract
In situ, ex situ, and microscopic Raman spectroscopy were used to examine the interactions of dilute chromate solutions with actively corroding pits in aluminum alloy used for aerospace applications (AA-2024-T3). Approximately in salt solution greatly reduces alloy corrosion, and soluble Cr(VI) is a major component of chromate conversion coatings (CCC). The interactions of dilute Cr(VI) with corroding alloy are likely to underlie the “self‐healing” property of CCCs which make them so useful. Raman microscopy revealed that was transported into an active pit and formed a chromate corrosion product (CCP) observable by its distinctive Raman band. This band was concentrated in pits and difficult to observe on the unpitted surface. Detailed spectroscopy of the CCP and several synthetic analogs revealed that the CCP is a mixed oxide of Al(III) and Cr(VI). Electrostatic bonding between the matrix and Cr(VI) is favored when the is cationic at low pH, while covalent bonding is also possible at higher pH. The spectra of the CCP formed in AA 2024 pits implies a local pH in the region of 6 to 7. The implications of these findings to the corrosion protection mechanism of Cr(VI) on aluminum are considered. © 1999 The Electrochemical Society. All rights reserved.

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